An improved Bayard-Alpert type vacuum ionization tube includes an electron collecting grid surrounding and coaxial with a centrally located positive ion collector. The grid has an inner diameter of approximately 0.5 inches and an electron emitting filament is positioned outside of the grid and spaced from the grid by approximately 0.2 inches. A grounded shield screen surrounds the grid outside of the filament and is coaxial with the collector. The tube has constant vacuum gauging sensitivity over the range from 10 1 to 5 X 10 10 torr.